Rather than saving Cosmic Radiance for the last moment and risking someone dying during the delay, it can be more beneficial to cast it as soon as you know a teamfight is guaranteed to start.

The story of Taric:

Taric is the Aspect of the Protector, wielding incredible power as Runeterra’s guardian of life, love, and beauty. Shamed by a dereliction of duty and exiled from his homeland Demacia, Taric ascended Mount Targon to find redemption, only to discover a higher calling among the stars. Imbued with the might of ancient Targon, the Shield of Valoran now stands ever vigilant against the insidious corruption of the Void.